1. Field of the Invention
This invention relates in general to offshore oil and gas floating platforms, and in particular to an apparatus for tensioning risers extending from a subsea wellhead to a floating platform.
2. Description of the Prior Art
Offshore production platforms must support production risers from oil or gas wells which extend to the platform from subsea wells. For platforms that are fixed to the ocean floor this is readily accomplished and is well known in the art. However, for subsea completions in deep water that require the use of floating platforms, such as tension leg platforms or semi-submersible platforms, supporting risers present significant problems. These platforms move under the influence of waves, wind, and current. Thus, the riser tensioning mechanism must permit the platform to move relative to the riser.
The riser tensioning mechanism must also maintain the riser in tension so that the weight of the riser is not transferred to the wellhead and so that the riser does not collapse under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser. Also, this force must be maintained within a narrow tolerance.
The use of a hydraulic cylinders attached between a platform and a riser to support the weight of the riser is well known in the art. However, due to the long life of production platforms, it has been necessary to have backup systems so that repairs can be made to the hydraulic cylinders in cases of failure. One such device is described in the U.S. Pat. No. 4,004,532 to Reynolds. That patent discloses an apparatus that requires a completely redundant backup system comprising one or more sets of two in-line hydraulic heave compensator cylinder units, one of the cylinder units being the primary heave compensator unit, the other serving as a backup unit in the event of primary unit failure. Each unit in that system has its own dedicated air/oil accumulator and its own independent air pressure vessel.
In another previously known device a control system is used to charge the hydraulic cylinders. When one pair of hydraulic cylinders experiences a reduction in pressure, the control system isolates and completely relieves the pressure in that pair, while the other cylinder pairs are charged to twice the normal pressure in order to maintain a constant tensioning force. This system, however, requires separate high pressure containers to charge each of the hydraulic cylinders. Also, there is a period of elapsed time that is required before the system can register the pressure deficiency and perform the necessary switching. During this time substantial bending moments are present in the riser which can result in damage to the riser or the support system.
Another tensioning system is disclosed in U.S. Pat. No. 4,362,438, which does not require connection between the hydraulic cylinders and separate control system. This patent describes an apparatus with hydraulic cylinders that have internal valves that open and close, allowing hydraulic fluid to travel through internal conduits to compensate for pressure changes caused by riser movement relative to the platform. This system, however, provides no protection against bending moment stresses caused by failure of hydraulic cylinders.